This invention relates to a parallel gripper which includes two gripping jaws moved simultaneously in opposite directions toward or away from one another by an electric motor, preferably a stepping motor via a common worm wheel or separate worm wheels of a worm gear drive. The shaft of each worm wheel is extended to form a spindle on which at least one spindle nut is threadedly mounted to travel parallel to the spindle axis as the spindle is rotated. A support is affixed to the spindle nut and prevented from rotation about the spindle axis. In case two supports are provided, they move on each side of the worm wheel on the spindle in opposite directions. Each gripping jaw is carried by a respective support. Further, for each gripping jaw a force-measuring device is associated, having strain gauges for detecting deformations.
German Offenlegungsschrift (application published without examination) 36 06 874 discloses a gripper having gripping jaws which, for adjusting the gripping opening, are shifted by a motor with the intermediary of a worm screw drive including threaded spindles. The motor, particularly a stepping motor, is supported against a biasing force for displacement parallel to the length direction of the worm screw. It is a disadvantage of this arrangement that an accurate measurement of the gripping force at each gripping jaw and an indication of the gripping position are not possible.
Further, German Offenlegungsschrift 39 05 656 discloses a parallel gripper having a spindle drive and presence monitoring means in which, with the aid of a special switching device connected with the spindle nut, a switching signal is emitted if, during the gripping process, the switch is actuated by a body and by the pressure applied thereby to the jaws. The switch is coupled to the motor control which generates control signals for the motor. In this prior art construction too, a measurement and regulation of the gripping force is not possible.
It is further known, such as disclosed, for example, in German Offenlegungsschrift 39 10 801 to indirectly determine the gripping force by means of the motor current. Such a method, however, is inaccurate and also, it cannot determine the gripping force at the individual jaws.
Further, U.S. Pat. No. 4,132,318 discloses a manipulating system including a two-jaw gripper wherein each jaw is coupled with a gripping jaw drive by means of a six-component force-and-torque system. Both measuring systems are asymmetrical; forces and torques are determined with the aid of measured deformations. Such a gripper is unstable; a reliable, positive gripping cannot be effected because the jaws are pivotal in all directions. Further, no overload safety device appears to be provided or is feasible.